Structure and Transport Properties of Dense Polycrystalline Clathrate-II (K,Ba)16(Ga,Sn)136 Synthesized by a New Approach Employing SPS

Tin clathrate-II framework-substituted compositions are of current interest as potential thermoelectric materials for medium-temperature applications. A review of the literature reveals different compositions reported with varying physical properties, which depend strongly on the exact composition as well as the processing conditions. We therefore initiated an approach whereby single crystals of two different (K,Ba)₁₆(Ga,Sn)₁₃₆ compositions were first obtained, followed by grinding of the crystals into fine powder for low temperature spark plasma sintering consolidation into dense polycrystalline solids and subsequent high temperature transport measurements. Powder X-ray refinement results indicate that the hexakaidecahedra are empty, K and Ba occupying only the decahedra. Their electrical properties depend on composition and have very low thermal conductivities. The structural and transport properties of these materials are compared to that of other Sn clathrate-II compositions.

Binary Alkali-Metal Silicon Clathrates by Spark Plasma Sintering: Preparation and Characterization

The binary intermetallic clathrates K_8-xSi₄₆ (x = 0.4; 1.2), Rb_6.2Si₄₆, Rb_11.5Si₁₃₆ and Cs_7.8Si₁₃₆ were prepared from M₄Si₄ (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb_11.5Si₁₃₆ was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si₂₀ dodecahedra, was found in all compounds. SPS preparation of Na₂₄Si₁₃₆ with different SPS current polarities and tooling were performed in order to investigate the role of the electric field on clathrate formation. The electrical and thermal transport properties of K_7.6Si₄₆ and K_6.8Si₄₆ in the temperature range 4–700 K were investigated. Our findings demonstrate that SPS is a novel tool for the synthesis of intermetallic clathrate phases that are not easily accessible by conventional synthesis techniques.

Distribution of CSL grain boundaries in oriented cemented carbides

The anisotropy of distributions of specific WC planes and coincidence site lattice (CSL) grain boundaries can be tailored by adjusting the composition of the WC–Co composite powder and its sintering parameters.

Thermoelectric Properties of Ga/Ag Codoped Type-III Ba₂₄Ge₁₀₀ Clathrates with in Situ Nanostructures

Because of the low thermal conductivity and high electrical conductivity, type-III Ba24Ge100 clathrates are potentially of interest as power generation thermoelectric materials for midto-high temperature operations. Unfortunately, their too high intrinsic carrier concentration results in a quite low Seebeck coefficient. To reduce the carrier concentration, we prepared a series of Ga/Ag codoped type-III Ba24Ge100 clathrate specimens by vacuum melting and subsequently compacted by spark plasma sintering (SPS). Doping Ga-Ag on the sites of Ge reduces the concentration of electrons and, at higher concentrations, also leads to the in situ formation of BaGe2 nanoprecipitates detected by the microstructural analysis. As a result of doping, the Seebeck coefficient increases, the thermal conductivity decreases, and the dimensionless figure of merit ZT reaches a value of 0.34 at 873 K, more than three times the value obtained with undoped Ba24Ge100.